Very cool topic
For me, I guess their are several reasons and motivations. First and foremost....It's something I love to do....but it's far more than that. Photography is something I love to do for other people. Photography is my full time job and source of income. I love creating photographs for other people, always striving to provide them with a unique product that exceeds their expectations.
I love the learning process....the more I learn, the more I realize I don't know. Once the technical side is understood, there is a never ending experimentation/learning process involved.
I embrace (as well as struggle with) the subjective aspect of photography.....I guess my ultimate goal would be to create a great photograph and be able to know it's great without getting the opinions of others. I'm not even sure this is possible. Any art form is so subjective that one persons awesome is another persons awful.
If I had to boil it down, I guess it would be the overall daily challenge that photography presents. Anyone can take a photo but the challenge to create a photograph that stands out above the rest is an awesome endeavour to embark upon.